There are many current and emerging technologies, including medical procedures, that benefit from the measurement of biopotentials. One of the most common procedures using biopotential measurement is probably the electrocardiogram (EKG), which is typically used to assess heart rhythms. While the capabilities of almost all aspects of the systems used to implement these measurements continue to improve, the somewhat limited performance of biopotential sensors has limited the applications in which biopotential measurement can be successfully performed.
With this in mind, biopotential sensors can generally be categorized as either invasive or non-invasive. Invasive sensors are typically implanted surgically and are generally used to isolate one or more specific potential sources such as the brain or the peripheral nervous system. Non-invasive sensors, on the other hand, have heretofore been applied to a surface such as the patient's skin or scalp. One type of non-invasive sensor, often referred to as a wet electrode, utilizes a conducting electrolyte, typically a gel, to electrically connect the electrode to the skin of the patient. This technique is currently the standard method used in clinical and research applications due to the relatively low cost of the electrode, its relatively long history of use, and the fact that the technique achieves a relatively good electrical contact between the electrode and the patient. There are, however, certain disadvantages with this technique. For some applications (e.g. an EKG), the technique requires the patient to disrobe and may require the skin to be shaved and prepped. Because of these requirements, procedures using wet electrodes are often time consuming, labor intensive and uncomfortable for the patient.
Another type of non-invasive sensor utilizes a surface electrode that does not require an electrolyte gel. These electrodes are referred to as active electrodes and typically employ an impedance transformation using active electronics. The active electrodes can be either insulated electrodes or dry electrodes (i.e. non-insulated). Typically, the active dry electrode is a conductive metal which is placed in direct contact with the skin and relies on a combination of resistive and capacitive coupling to the local skin potential to receive its signal. On the other hand, the insulated electrode relies entirely on capacitive coupling for this purpose.
Heretofore, active dry and insulated electrodes have not typically exhibited the same consistency and signal to noise ratio (SNR) as wet electrodes. Although considerable efforts have been made to improve insulated electrodes by using coatings with a high dielectric constant to improve the capacitance to the skin, there are still substantial limitations associated with currently available non-invasive sensors, including those that use insulated electrodes. For example, currently available sensors are strongly affected by small displacements away from the skin. In greater detail, as implied above, these sensors must be positioned either directly in contact with, or extremely close to, the skin. For dry electrodes, the signal is completely lost if the electrode is moved away from the skin by only a few microns. For capacitive electrodes, the effect of electrode displacement can be estimated by first recognizing that the capacitive coupling is similar to that of a parallel plate capacitor in which the skin acts as one capacitor plate and the electrode acts as the other plate. Based on this model, the coupling is proportional to the inverse of the separation distance between the sensor and the skin. In numerical terms, the coupling is typically reduced by a factor of about 10 as the electrode moves from a position of contact with the skin to a stand-off distance of only about 100 μm.
Heretofore, efforts to reduce the effects of small sensor displacements have involved using capacitive sensors with multiple sensing regions. Relatively complicated processing circuitry is then integrated with the active electronics connected to the electrode to switch between signals from sensing regions having adequate electrode coupling and signals from sensing regions where the electrode coupling is insufficient. While in theory this approach can reduce the effect of sensor motion in the final measurement, it has not been widely adopted.
In contrast to the above-described techniques, the present invention contemplates a sensor capable of measuring an electric potential in free space. More specifically, the present invention contemplates the measurement of an electric potential with an electrode that is not necessarily in direct contact with, or even extremely close to, the biopotential source. Unlike the sensors described above, for an electrode that is spaced from the biopotential source, the effect of a small displacement between the sensor and biopotential source is minimal. In greater detail, it is known that the electric potential, E, produced in a uniform medium by a simple source of electrical potential such as a charge decreases in accordance with the relationship, E∝1/distance2. Thus, for a sensor having an electrode spaced at a distance of 2 cm from a dipole source, the relative change in signal for a 100 μm displacement away from the source is only about 1%.
Unfortunately, the measurement of the free-space electric potential (i.e. measurement with an electrode that is not necessarily in direct contact with, or extremely close to the biopotential source) cannot be performed effectively with currently available sensors. Specifically, conventional insulated electrodes typically rely on a relatively large mutual capacitance between the electrode sensing area and the surface of the skin. For an electrode pressed directly on the skin, the resulting capacitance can be as high as 0.1 μF/cm2. With this relatively large capacitance, it is relatively simple to construct an amplifier using modern semiconductor technology with sufficiently high input impedance and input bias current path impedance to effectively amplify the signal from such an electrode. However, to measure a potential in free space, one is limited to the free space capacitance of the sensing part of the electrode. This capacitance is typically about 1 pF multiplied by the average radius of the electrode measured in centimeters, thus the capacitance is only about 1 pF for most practical applications.
Typically, to measure the free space potential with an adequate sensitivity for most biopotential measurement applications, it is preferable that the sensor electrode measure the free space electric potential with a noise floor of below approximately 20 μV/Hz0.5 at 1 Hz. Further, to achieve a voltage noise of about 20 μV/Hz0.5 at 1 Hz requires an input current noise on the order of about 1 fA/Hz0.5 at 1 Hz. In addition, coupling to the small free space capacitance of the sensor electrode (i.e. 1 pF) generally requires that the input impedance of the first stage sensor electronics be of the same order as the impedance of the sensing layer. This translates to an input resistance of about 100 GΩ or higher, and an input capacitance of about 10 pF or less. Also, the circuit used to provide the input bias current to the first stage electronics should have an impedance to ground of the same order as the amplifier input impedance, and stray capacitances at the input to the system should be less than the amplifier input capacitance. These specifications delineate a threshold sensitivity for measuring a typical biopotential at a standoff distance. To measure smaller than average biopotential signals, such as those that arise in an electroencephalogram (EEG), it is desirable that the amplifier input impedance be even higher, and the first stage amplifier current noise be even lower.
In light of the above, it is an object of the present invention to provide systems and methods suitable for effectively measuring a biopotential signal that are operable with a relatively weak coupling between the sensor electrode and the biopotential source. It is yet another object of the present invention to provide systems and methods for measuring biopotentials that minimize the effect of small movements between the sensor electrode and the biopotential source during measurement. It is another object of the present invention to provide systems and methods for non-invasively measuring biopotential signals through a patient's clothing. Yet another object of the present invention is to provide systems and methods for measuring biopotentials which are easy to use, relatively simple to implement, and comparatively cost effective.